Many batteries require a charge curve which provides a region of constant current output during the final hours of charging. This constant current, usually approximately 2-5 amps/100 amp hours of battery capacity, is provided by raising or lowering the output voltage of the charger to maintain the desired current. Regulating the output voltage is frequently accomplished by means of a feedback control circuit which incorporates conventional electronic components such as a silicon controlled rectifier (SCR), a magnetic amplifier, transistors, and possibly a microprocessor, for providing a constant voltage or constant current at the output. The control circuit constantly monitors the output voltage and output current and provides correction to make the charger perform in accordance with a given reference output curve.
Other types of battery chargers incorporate a ferroresonant battery charging circuit employing only a ferroresonant transformer and diode rectifiers to produce the DC output. Referring to FIG. 1, there is shown in simplified schematic diagram form a prior art battery charger 10 incorporating a ferroresonant transformer 12. An AC input is provided via input terminals 16a and 16b to a primary winding 22 (which is shown as two windings in the figure) of the ferroresonant transformer 12. The ferroresonant transformer 12 further includes a magnetic core 24 as well as a secondary output winding 26 which is also shown as two separate windings. The secondary output winding 26 provides a DC output to terminals 50a and 50b via first and second rectifying silicon diodes 32 and 34 and a DC fuse 36. Also provided in the DC output circuit is a surge protector 30 and an ammeter 38 with shunt. The DC output terminals 50a and 50b are adapted for connection to a storage battery 14 (shown in dotted-line form) for the charging thereof. Transformer 12 further includes a resonating winding 28 in circuit with a resonating capacitor 54. A set of magnetic shunts 52 separates magnetic paths of the secondary winding 26 and the resonating winding 28 from the magnetic path of the transformer's primary winding 22 on the magnetic core 24. The combination of the resonating winding 28 and resonating capacitor 54 functions to regulate the output current at a constant value. When the value decreases to a small portion of the rated current of the transformer, the output voltage increases limiting regulation of the output current at low values by transformer 12. Battery charger 10 further includes a timer circuit 40 including a control transformer 42, a time clock 44, a switch 46 and an input contactor 48. Timer circuit 40 is coupled between the first and second pairs of AC input terminals 16a and 16b and AC contactors 18 and 20. Time clock 44 is set and begins counting when a battery 14 is connected to DC output terminals 50a and 50b and switch 46 is closed. When time clock 44 times-out after a predetermined time interval, switch 46 opens turning off the battery charger 10 by electrically decoupling the AC contactors 18 and 20 from the AC input terminals 16a and 16b. Timer circuit 40 thus controls the length of time that battery charger 10 is on.
Although prior art ferroresonant battery chargers do not have the complexity and associated costs of feedback-type battery charges, they are not without limitations. The characteristics of the output voltage and current curves are determined by the charger's ferroresonant transformer design. A ferroresonant transformer has an inherent current limit for regulation. At currents lower than this inherent limit, the output voltage undergoes a corresponding increase and low current regulation is limited. Charging a battery with a finish current which is not precisely regulated uses more electrolyte and decreases battery lifetime. In addition, the inability to regulate at low finishing currents makes the battery charger unsuitable for charging batteries with low or high specific gravity using the same charger and batteries which are subjected to hot or cold charging environments where the battery terminal voltages required to charge the battery change with temperature.
The present invention addresses the aforementioned limitations of the prior art by providing a battery charger with a ferroresonant transformer which is capable of providing a precisely regulated output current even at the low end limit of the charger such as during the final hours of charging when currents typically on the order of 5% of the rated output current are required.